<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .div1 {
            background-color: rebeccapurple;
        }
    </style>
</head>

<body>

    <div class="div1">我是div1</div>
    <div>我是div2</div>
    <hr>
    <h2>
        normal flow
    </h2>
    <p>
        [说明]:块级元素的默认宽度是父元素的100%，默认高度是子元素撑开的。
    </p>

</body>

</html>